In general, the static magnetic fields in MRI systems may be more than about 0.5 T; some such systems have static magnetic fields greater than 3 T. These magnetic fields will attract metal objects in the vicinity of the static magnets. If the object is in the vicinity of the MRI's opening, such as an opening to allow entrance of a subject to be analyzed, the object can become very difficult to remove from the interior of the MRI. Worse, in an MRI located within the hospital, for imaging patients, such a magnetic fringing field can be physically dangerous to the patient, if not fatal.
Fringe fields from a high field strength can extend to a distance determines roughly by the equation X=αB0, where α is a constant dependent on the magnet bore size and magnet configuration. An unshielded 1.5 T magnet has a 1 mT fringe field at a distance of ˜9.3 m, a 0.5 mT field at 11.5 m, and a 0.3 mT field at 14.1 m from the center of the magnet. To reduce the extent of the magnetic fringe field, passive (such as thick metal walls close to the magnet) and active (electromagnet systems placed in the magnet housing, superconductive magnets U.S. Pat. Nos. 5,883,558, 5,565,831 magnetic shielding are applied. The distance of a shielded fringe field surrounding the MRD opening can be reduced to about 2 meters or less.
Despite the achievable reduction of the magnetic fringe field boundaries, the existence of such field provides room for the above mentioned dangers in the MRD vicinity. Therefore, there is a long-felt need to provide an automatic and safe means of preventing loose metal objects from entering the fringe field and the MRD bore and protecting the patients from metallic instruments.